CoFeSiB/Pt multilayers exhibiting perpendicular magnetic anisotropy

ABSTRACT

Provided is a magnetic anisotropy multilayer including a plurality of CoFeSiB/Pt layers used in a magnetic random access memory. The magnetic anisotropy multilayer includes a first Pt/CoFeSiB layer, and a second Pt/CoFeSiB layer formed on the first Pt/CoFeSiB layer.

PRIORITY

This application claims priority under 35 U.S.C. §119 to a Koreanapplication filed in the Korean Intellectual Property Office on Nov. 28,2006 and allocated Serial No. 2006-118143, the contents of which areincorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to a magnetic anisotropymultilayer used in a magnetic random access memory, and moreparticularly, to a magnetic anisotropy multilayer including a pluralityof CoFeSiB/Pt layers.

2. Description of the Related Art

A magnetic random access memory (hereinafter, referred to as a magneticmemory) is being used in military applications such as a missile, aspacecraft, and the like. The magnetic memory has advantages of volatiledevices such as a dynamic random access memory (DRAM) and a staticrandom access memory (SRAM), i.e., both high integration degree of aDRAM and high-speed performance of an SRAM. In addition, the magneticmemory has lower power consumption than a nonvolatile type flash memory,and it has great number of repetition times of record reproduction.Therefore, it is considered as a substitution for the existing memorythat has been used in a mobile phone, a computer and a network. Also,attempts are being made to apply the magnetic memory to a radiofrequency identification (RFID) tag requiring low price and volatility,and further there is a great likelihood that it can be applied to arobot for factory automation.

The magnetic memory is a magnetic memory device having magnetic tunneljunctions (MTJs) based on tunneling magnetoresistance (TMR). Themagnetic memory can input data by using spin directions caused byself-revolution of electrons in the device. In detail, the resistance ofthe magnetic memory is changed according as the spin directions ofadjacent magnetic layers become parallel or anti-parallel, and the spindirection can be controlled to be parallel or anti-parallel by applyinga magnetic field from the exterior. By using this property, it ispossible to input data in the magnetic memory.

In general, the MTJ is configured in the shape of a sandwich where aninsulating layer (generally, Al₂O₃ or MgO layer) as a tunneling barrieris interposed between two ferromagnetic layers. A current flowsperpendicular to each layer. One of the two ferromagnetic layers is apinned layer acting as a reference layer and the other one is a freelayer for magnetic recording or sensing. In case that the spindirections of the two ferromagnetic layers are parallel with each otherwhen current flows, the resistance becomes small so that the tunnelingprobability of current becomes great. On the contrary, when the spindirections of the two ferromagnetic layers are anti-parallel, theresistance becomes large, which results in reducing the tunnelingprobability of current. For ultra high integration of the magneticmemory, it should be necessary to form submicron memory cells. If a unitMTJ shrinks in size and an aspect ratio of the cell is also reduced forachieving the high integration degree of the magnetic memory,multi-domains or a vortex is formed inside a magnetic substance of theMTJ due to strong diamagnetic field. This leads to an unstablecell-switching phenomenon, which decreases a writing margin.

When fabricating the cell with high aspect ratio, such a multi-domainstructure is not formed in virtue of shape magnetic anisotropy but it isdifficult to achieve high integration. Moreover, this requires highswitching magnetic field so that it is impossible to highly integratethe device after all.

To overcome such a problem, a perpendicular magnetic anisotropy MTJ hasbeen developed (Naoki Nishimura et al., J. Appl. Phys., vol. 91, p.5246. 2002). In Nishimura et al., the MTJ was fabricated usingrare-earth and transition-metal alloys such as TbFeCo and GdFeCo, whichhas been well known as perpendicular magnetic anisotropy material, as afree layer and a pinned layer, respectively. The magnetoresistance ratioof this MTJ was 55%. In addition, it was confirmed that there was nosusceptibility distortion at the perpendicular magnetic anisotropy MTJthrough a magnetic force microscope (MFM). However, Tb and Gd used inthis experiment are disadvantageous in terms of low corrosion resistanceand difficulty in property control, and thus it is not easy to put theseelements into the practical use. Therefore, for practical use of theperpendicular magnetic anisotropy MTJ (in short, pMTJ), it is necessaryto develop new perpendicular magnetic anisotropy material.

The perpendicular magnetic anisotropy layers that have been researchedwere developed in order that they may be substituted for longitudinalmagnetic storage media which will encounter the limitation of highdensity. The material exhibiting the perpendicular magnetic anisotropyis CoCr-based alloy layer, Co/Pt, or Co/Pd multilayer, wherein thismaterial should meet specific physical properties such as highperpendicular magnetic anisotropy, high coercivity and high remanentmagnetization.

However, as the magnetic memory needs rapid switching and low powerconsumption, low coercivity and high magnetic anisotropy for theincrease of a writing margin are required. In addition, it is requiredto maintain a remanent magnetization to be similar to a saturationmagnetization and simultaneously to be low for improving the sensitivityof the magnetic memory switching operation.

In order to employ a perpendicular magnetic tunnel junction (pMTJ) thathas been actively researched to meet the demand for high integration ofthe magnetic memory, the perpendicular magnetic anisotropy layerexhibiting low coercivity, low saturation magnetization and highmagnetic anisotropy should be used. That is, the perpendicular magneticanisotropy MTJ formed of the above-listed perpendicular magneticanisotropy material, in which the magnetization direction isperpendicular to the layer surface, has low saturation magnetization andno magnetic distortion at an edge of the layer, which will make itpossible to realize the high integration of the magnetic memory.

Accordingly, it is required a perpendicular magnetic anisotropy layerhaving a low coercivity and a low saturation magnetization similar to aremanent magnetization, and capable of minimizing power consumption aswell.

SUMMARY OF THE INVENTION

An object of the present invention is to substantially solve at leastthe above problems and/or disadvantages and to provide at least theadvantages below. Accordingly, an object of the present invention is toprovide a magnetic anisotropy layer having a low coercivity and a lowsaturation magnetization or smaller similar to a remanent magnetization.

Another object of the present invention is to provide a CoFeSiB/Ptmagnetic anisotropy layer having a low coercivity (20 Oe or smaller) anda low saturation magnetization (172 emu/cm³) similar to a remanentmagnetization (squareness≈1).

Further another object of the present invention is to provide a magneticanisotropy layer configured with CoFeSiB/Pt multilayers having a lowcoercivity (20 Oe or smaller) and a low saturation magnetization (172emu/cm³) similar to a remanent magnetization (squareness≈1).

According to one aspect of the present invention, a perpendicularmagnetic anisotropy multilayer includes: a first Pt/CoFeSiB layer; and asecond Pt/CoFeSiB layer formed on the first Pt/CoFeSiB layer.

According to another aspect of the present invention, a perpendicularmagnetic tunnel junction (PMTJ) including a free layer and a pinnedlayer separated by a non-magnetic spacer layer (tunnel barrier), whereinthe free layer includes: a first Pt/CoFeSiB layer; and a secondPt/CoFeSiB layer formed on the first Pt/CoFeSiB layer.

In addition, there may be provided other embodiments having structuresdifferent from the above magnetic anisotropy multilayer, or otherembodiments realized by modifying and adding elements.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and advantages of the presentinvention will become more apparent from the following detaileddescription when taken in conjunction with the accompanying drawings inwhich:

FIG. 1 is a schematic view illustrating a multi-stacked structure ofCoFeSiB/Pt perpendicular magnetic anisotropy layer according to thepresent invention;

FIG. 2 is a graph illustrating the comparison of magnetic propertiesbetween an existing Co/Pt layer and the CoFeSiB/Pt formed according tothe present invention;

FIG. 3 is a graph illustrating variations of coercivity and saturationmagnetization of the multilayer depending on the variation of Ptthickness (t) in Si/SiO₂/Ta 50/Pt 4/[Pt (t)/CoFeSiB 3]×5/Ta 50(thickness unit: Angstrom);

FIG. 4 is a graph illustrating variations of coercivity and saturationmagnetization of the multilayer depending on the variation of number ofrepetition times (n) in Si/SiO₂/Ta 50/Pt 4/[Pt 8/CoFeSiB 3]×n/Ta 50(thickness unit: Angstrom); and

FIG. 5 is a graph illustrating variations of coercivity and saturationmagnetization of the multilayer depending on the variation of CoFeSiBthickness (x) in Si/SiO₂/Ta 50/Pt 4/[Pt 8/CoFeSiB (x)]×5/Ta 50(thickness unit: Angstrom).

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferred embodiments of the present invention will be described hereinbelow with reference to the accompanying drawings. In the followingdescription, well-known functions or constructions are not described indetail since they would obscure the invention in unnecessary detail.

A multi-stacked structure of a perpendicular magnetic anisotropymultilayer according to the present invention is represented asSi/SiO₂/Ta/Pt/[Pt/CoFeSiB]×n/Ta, where Si denotes a substrate, SiO₂ is anative oxide layer formed on the substrate, and Ta and Pt layers arebuffer layers. The other Ta layer is a capping layer. The subject of thepresent invention is [Pt/CoFeSiB] layer exhibiting perpendicularmagnetic anisotropy. That is, the CoFeSiB layer is used in the presentinvention instead of the Pt/Co or Pt/CoFe layer that was used for theexisting perpendicular anisotropy multilayer.

Hereinafter, preferred embodiments of the present invention are intendedto provide or will be described in detail with reference to accompanyingdrawings.

FIG. 1 is a schematic view illustrating a multi-stacked structure ofCoFeSiB/Pt perpendicular magnetic anisotropy layer formed according tothe present invention.

The perpendicular magnetic anisotropy layer of FIG. 1 formed accordingto one embodiment of the present invention can be expressed asSi/SiO₂/Ta 50/Pt 4/[Pt (t₁)/CoFeSiB (t₂)]×n/Ta 50 in consideration ofrespective thicknesses, where thickness unit is angstrom. Herein, ndenotes the number (i.e., the number of repetition times) of Pt(t₁)/CoFeSiB (t₂) layers, and t₁ and t₂ denotes thicknesses of Pt andCoFeSiB, respectively. Further, Si denotes the substrate, and SiO₂ layerof which a thickness is considered to be insignificant is an oxide layerformed on the substrate.

Here, the CoFeSiB has a composition of Here,Co_(84.8)Fe_(5.96)Si_(6.24)B_(3.04)˜Co_(70.5)Fe_(4.5)Si₁₅B₁₀ (in atomicpercent) where t₁, t₂ and n are variables that can be variously changed.

A direct current (DC) magnetron sputtering method is used for depositingthe multilayer of the present invention, where base pressure is lessthan 5×10⁻⁸ Torr. The thickness of the layer is controlled throughdeposition time. The Pt and CoFeSiB layers are repetitively deposited tothereby form a perpendicular magnetic anisotropy in virtue of surfaceanisotropy at an interface. According to measurement results of magneticproperties, the coercivity is about 20 Oe, the saturation magnetizationis about 170 emu/cm³, and the perpendicular magnetic anisotropy is about5×10⁵ erg/cm³˜5×10⁶ erg/cm³, in the layer structure of Si/SiO₂/Ta 50/Pt4/[Pt 8/CoFeSiB 3]×4/Ta 50 (thickness unit: Angstrom). Here, it ispreferable that the number of repetition times be 3 or 4. As the numberof repetition times increases, the coercivity also increases, which isnot preferable.

FIG. 2 is a graph illustrating the comparison of magnetic propertiesbetween an existing Co/Pt multilayer and the CoFeSiB/Pt multilayerformed according to the present invention.

More specifically, FIG. 2 is a graph illustrating the comparison resultsof magnetic properties between the existing [Co/Pt] multilayer and theinventive [CoFeSiB/Pt] multilayer, in which they have the same structureand layer thickness. That is, the layer structure is Si/SiO₂/Ta 50/Pt4/[Pt 8/CoFeSiB or Co 3]×5/Ta 50 (thickness unit: Angstrom), and themagnetic property is measured using a vibrating sample magnetometer(VSM).

According to the results, the [CoFeSiB/Pt] multilayer of the presentinvention has one-tenth the coercivity and one-third the saturationmagnetization of the existing [Co/Pt] multilayer. Thus, when the[CoFeSiB/Pt] multilayer of the present invention is applied to a freelayer of pMTJ, it shows good switching characteristic.

FIG. 3 is a graph illustrating variations of coercivity and saturationmagnetization of the multilayer depending on the variation of Ptthickness (t) in the layer structure of Si/SiO₂/Ta 50/Pt 4/[Pt(t)/CoFeSiB 3]×5/Ta 50 (thickness unit: Angstrom).

Referring to FIG. 3, it can be understood that the coercivity and thesaturation magnetization are varied with the variation of the Ptthickness (t₁) in the layer structure of Si/SiO₂/Ta 50/Pt 4/[Pt(t)/CoFeSiB 3]×5/Ta 50 (thickness unit: Angstrom). That is, it can beunderstood that the dipole-interaction between magnetic layers decreaseswith the increase of the Pt thickness, and particularly, the coercivityis abruptly reduced if the Pt thickness exceeds to 12 Å. In addition, itcan be also understood that the saturation magnetization decreases asthe Pt thickness increases. It is preferable that the coercivity be lessthan 20 Oe in order that the multilayer may be used for a perpendicularmagnetic tunneling junction (pMTJ).

FIG. 4 is a graph illustrating variations of coercivity and saturationmagnetization of the multilayer depending on the variation of number ofrepetition times (n) in the layer structure of Si/SiO₂/Ta 50/Pt 4/[Pt8/CoFeSiB 3]×n/Ta 50 (thickness unit: Angstrom). Predictably, as thenumber of repetition times increases, it is understood that thecoercivity increases and the saturation magnetization increases to apredetermined degree and then becomes constant after a predeterminedvalue. Since the coercivity should be less than 20 Oe in order that themultilayer may be used for the pMTJ, it is preferable that the number ofrepetition times be 3 or 4.

FIG. 5 is a graph illustrating variations of coercivity and saturationmagnetization of the multilayer depending on the variation of CoFeSiBthickness (x) in Si/SiO₂/Ta 50/Pt 4/[Pt 8/CoFeSiB (x)]×5/Ta 50(thickness unit: Angstrom) formed according to the present invention.

Referring to FIG. 5, it is understood that the coercivity and thesaturation magnetization are varied with the variation of CoFeSiBthickness (t₂) in the Si/SiO₂/Ta 50/Pt 4/[Pt 8/CoFeSiB t₂]×5/Ta 50. Whenthe thickness of the CoFeSiB layer is 6 Å or smaller, the coercivityincreases because the perpendicular magnetic anisotropy increases withthe increase of thickness. However, when the thickness of the CoFeSiBlayer is greater than 6 Å, the coercivity of the perpendicular componentdecreases because a bulk anisotropy increases and the horizontalmagnetic component increases correspondingly. In case of the saturationmagnetization, the perpendicular magnetic anisotropy increases as thethickness of the perpendicular magnetic anisotropy layer increases oncondition that the thickness of the CoFeSiB layer is 6 Å or smaller.However, it can be understood that the saturation magnetization does notincrease any more when the thickness is greater than a predeterminedvalue. It is preferable that the coercivity be less than 20 Oe in orderthat the multilayer may be used for the pMTJ. Thus, it is understoodthat the CoFeSiB layer has the thickness of 3 Å or smaller in case thatPt has a thickness of 8 Å.

According to the present invention, it is possible to provide a[Co_(84.8)Fe_(5.96)Si_(6.24)B_(3.04)/Pt]×n magnetic anisotropymultilayer having low coercivity, high perpendicular magneticanisotropy, and low saturation magnetization which is similar toremanent magnetization. Accordingly, it is also possible to form ahighly integrated magnetic memory. Here,Co_(84.8)Fe_(5.96)Si_(6.24)B_(3.04)˜Co_(70.5)Fe_(4.5)Si₁₅B₁₀ can be usedfor the present invention.

While the invention has been shown and described with reference tocertain preferred embodiments thereof, it will be understood by thoseskilled in the art that various changes in form and details may be madetherein without departing from the spirit and scope of the invention asdefined by the appended claims.

What is claimed is:
 1. A perpendicular magnetic anisotropy multilayercomprising: a first Pt/CoFeSiB layer; and a second Pt/CoFeSiB layerformed on the first Pt/CoFeSiB layer, wherein the first and secondPt/CoFeSiB layers are perpendicular anisotropy magnetization films andthe perpendicular magnetic anisotropy of the multilayer is about 5×10⁵erg/cm³ to about 5×10⁶ erg/cm³.
 2. The perpendicular magnetic anisotropymultilayer of claim 1, wherein the CoFeSiB layer has a composition ofCo_(84.8)Fe_(5.96)Si_(6.24)B_(3.04)CoO_(70.5)Fe_(4.5)Si₁₅B₁₀.
 3. Theperpendicular magnetic anisotropy multilayer of claim 1, wherein athickness ratio between the first Pt/CoFeSiB layer and the secondPt/CoFeSiB layer is 1:1.
 4. The perpendicular magnetic anisotropymultilayer of claim 1, further comprising a third Pt/CoFeSiB layerformed on the second Pt/CoFeSiB layer.
 5. The perpendicular magneticanisotropy multilayer of claim 4, further comprising a fourth Pt/CoFeSiBlayer formed on the third Pt/CoFeSiB layer.
 6. The perpendicularmagnetic anisotropy multilayer of claim 5, wherein a coercivity of theperpendicular magnetic anisotropy multilayer is 20 Oe or smaller.
 7. Theperpendicular magnetic anisotropy multilayer of claim 5, wherein theCoFeSiB layer has a thickness of 3 Å or smaller when the Pt layer has athickness of 8 Å.
 8. The perpendicular magnetic anisotropy multilayer ofclaim 5, wherein the Pt layer has a thickness of 14 Å or greater whenthe CoFeSiB layer has a thickness of 3 Å.